<!DOCTYPE HTML>
<style>
::-webkit-scrollbar {
    width: 12px;
    height: 12px;
    background: rgba(255,0,0,0.8);
}
body {
    width: 1000px;
    height: 1000px;
}
div {
    width: 100px;
    height: 100px;
    overflow: scroll;
}
</style>
<iframe src="resources/scrollable-iframe-customscrollbar-windowinactive-expected.html"></iframe>
<div><div></div></div>